The simplest way i can think of to explain RAM frequency is like this.
Imagine you have 3 cars.
A toyota corolla (cheap), a ford mustang(little expensive) and a ferrari (very expensive)
When you drive them on the highway you have to follow the speed limit 110 km/h
doesn't matter what car you get, all of them MUST obey the speed limit.
this is how ram works.

you can buy 1333MHz ram(cheap) or 1600MHz ram (little expensive) or 2666MHz ram(very expensive)
by default (no overclocking) all of them run at 1333MHz - this is the default DDR speed (your normal speed limit)

if you want a ferrari to go at 250 km/h you have to step on the gas.
if you want your 2666MHz ram to operate at 2666MHz, you have to overclock the system.
by default your 2666MHz ram will operate at 1333MHz when plugged into a system and it will have no performance benefits over regular ram operating at 1333MHz

its like having a ferrari driving at 110km/h on the highway, yes you will look good getting there but the corolla will get you there in exactly the same time. The difference only comes into play if you actually speed or in the RAM's case, you overclock.

The simplest way i can think of to explain RAM frequency is like this.
Imagine you have 3 cars.
A toyota corolla (cheap), a ford mustang(little expensive) and a ferrari (very expensive)
When you drive them on the highway you have to follow the speed limit 110 km/h
doesn't matter what car you get, all of them MUST obey the speed limit.
this is how ram works.

you can buy 1333MHz ram(cheap) or 1600MHz ram (little expensive) or 2666MHz ram(very expensive)
by default (no overclocking) all of them run at 1333MHz - this is the default DDR speed (your normal speed limit)

if you want a ferrari to go at 250 km/h you have to step on the gas.
if you want your 2666MHz ram to operate at 2666MHz, you have to overclock the system.
by default your 2666MHz ram will operate at 1333MHz when plugged into a system and it will have no performance benefits over regular ram operating at 1333MHz

its like having a ferrari driving at 110km/h on the highway, yes you will look good getting there but the corolla will get you there in exactly the same time. The difference only comes into play if you actually speed or in the RAM's case, you overclock.

There are two main methods to speed up the memory from the system default. One is to run a faster clock speed, the other is to use the same clock speed, but use fewer cas latency cycles to access the ram, for example, from CL9 (9-9-9-24) down to CL7 (7-7-7-20), or a combination of both. Here'a link to the previous thread on this:

Applying more voltage than spec will reduce the lifespan of your components. How much? that depends on the amount you over volt. You will need to do some research to find out specifics. But in most cases, a CPU that may last for 8-10 years will only last for 5-6 years. Doesn't really make that big of a difference given most people upgrade their systems in 3-4 years.

Over volting also carries the danger of burning out the component in question but this is rare and doesn't happen very often.

Your main enemy is heat. Overclocking = more heat generation. Small overclocks (5%) can be done with stock cooling. The overall heat generation doesn't increase enough to exceed the capacity of your heatsink/fan but if you are trying to achieve a nominal overclock, you will need to invest in an aftermarket cooling solution (new heatsink/fan)